Composite thermal interface material system and method using nano-scale components

ABSTRACT

A method of manufacturing a thermal interface material, comprising providing a sheet comprising nano-scale fibers, the sheet having at least one exposed surface; and stabilizing the fibers with a stabilizing material disposed in at least a portion of a void space between the fibers in the sheet. The fibers may be CNT&#39;s or metallic nano-wires. Stabilizing may include infiltrating the fibers with a polymerizable material. The polymerizable material may be mixed with nano- or micro-particles. The composite system may include two films, with the fibers in between, to create a sandwich. Each capping film may include two sub films: a palladium film closer to the stabilizing material to improve adhesion; and a nano-particle film for contact with a device to be cooled or a heat sink.

RELATED APPLICATION

The present application claims benefit of priority from U.S. ProvisionalPatent Application No. 61/037,125, filed Mar. 17, 2008, and from U.S.Provisional Patent Application No. 61/037,132, filed Mar. 17, 2008, eachof which is expressly incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of thermal interface materials (TIMs)for use in electronic devices or other thermal management applicationsthat require rapid dissipation of heat. Examples of devices needing TIMsinclude computers, telecommunications, space, military, and medicalapparatus.

2. Related Art

Power dissipation in electronic devices is projected to increasesignificantly over the next ten years to the range of 100-150 Watts percm² for high performance applications¹. This increase in powerrepresents a major challenge to systems integration since the maximumdevice temperature needs to be around 100° C. An additional concern isthat leakage currents may also significantly increase as theinterconnect size continues to decrease into the nanometer realm.Leakage currents will increase the power dissipation levels well beyondthe 150 W/cm² range. Thermal management is a major hurdle in thedevelopment of faster processors. In a typical chip heat sink assembly,the highest resistance to heat flow comes from the thermal interfacematerial. Typically, the thermal conductivity of a thermal interfacematerial ranges from 1-4 W/mK. One of the ways to increase thermalperformance is to improve the thermal conductivity of the thermalinterface material. Many concepts have emerged for increasing thethermal conductivity of thermal adhesives and pastes. A widely usedapproach is to add micron size, highly conductive filler particles inthe matrix of the thermal interface material. Another alternative is touse carbon nanotubes. Nanotubes have unique properties as discussed byIijima²⁰ and Berber et al.³ have reported nanotubes have measured highelectrical and thermal conductivities (around 6600 W/mK at roomtemperature) for carbon nanotubes. These can be placed in the thermalinterface material to provide a low heat resistance path through thethermal interface material, significantly improving the thermalconductivity of the TIM. See, Doctoral Dissertation of Anand HasmukhDesai “Thermal Management Of Small Scale Electronic Systems”, BinghamtonUniversity, State University of New York, 2006, the entirety of which isexpressly incorporated herein by reference.

Typical thermal interface materials used in production today includethermal greases and adhesives, thermal gels, phase change materials, andlow melt point solders such as Indium². The thermal conductivity forthese materials ranges from about 3 (grease and adhesives) to 30(solders) W/mK. The minimum required thickness of the thermal interfacedirectly impacts the resistance, and varies considerably between thesematerials. For example, solder TIM solutions need to be considerablythicker than thermal grease due to thermo-mechanical issues.

Carbon nanotubes (CNTs) are promising new materials exhibitingextraordinary thermal properties, when grown on a device requiring athermal interface. Theoretical calculations predict an unusually highvalue of phonon-dominated thermal conductivity at ca. 6600 W/mK^(3,4),while experimental measurements on individual CNTs confirms a range of3000-8000 W/(mK) at the room temperature^(5, 6). While the exact valuesand their validation are still under debate, there is little doubt thatthe extremely high thermal conductivity of CNTs offers the possibilityof using CNTs as TIM in electronics packaging to satisfy the increasingpower dissipation challenge.

SUMMARY AND OBJECTS OF THE INVENTION

Challenges in designing a thermal interface material arise due tointerfacial resistances, which are a function of surface conditions(devices and heat-sinks or heat spreaders), material properties of theTIM, and the assembly processes used to deposit the TIM and get it tothe design point thickness. The interfaces are particularly problematicsince they represent a transition between different materials and maycontain air voids and other defects. Interfaces typically result insignificant thermal resistances. A holistic system level optimization ofthe heat flow path between the device and the heat sink or heat spreaderis desirable. The system level design should account for the thermal,mechanical and chemical properties of the TIM material, the matingsurfaces and the effects of the assembly processes.

Prior concepts of using CNTs involve growing CNTs directly on a deviceto be cooled. This method tends to be cost-prohibitive.

Thermal interface materials are particularly vulnerable to damage undersome application conditions because they may interface between materialswith significantly different thermo mechanical properties such ascoefficient of thermal expansion and modulus of elasticity. Thesedifferences in properties mean that during thermal cycles such as thosethat arise when a machine is turned on and off, the TIM is subjected toa mechanical load. In many applications, such cycles occur for thousandsof times during the life of the product. The thermal interface materialmust be able to withstand such cycling without sustaining anysignificant damage.

Another issue to be addressed is the impact of the thermal interfacematerial on overall system cost. While the cost of the interfacematerial may be small, if it requires special assembly or handlingprocesses that may slow down the overall assembly process then that maysignificantly impact the overall system cost.

Recent studies on both randomly filled CNT composites⁷⁻⁹ and verticallyaligned CNT (VACNT) arrays¹⁰⁻¹² have shown less than idealcharacteristics. Typically, at most a few folds increase in thermalconductivity from that of the base materials has been observed. Aproblem has been the low conductance at filler/matrix and TIM/solidsinterfaces.

Additional issues in the use of CNTs as TIM's include:

-   -   Aligning the CNTs or other nano-scale fibers in the direction of        transport. In the case of a microprocessor, for example,        alignment is preferably perpendicular or near perpendicular to        the microprocessor surface, so that the CNT's transport heat to        the next level, namely a heat spreader or a heat sink;    -   Attachment between two surfaces to achieve heat transport        between those surfaces, such that interfacial resistances        between the CNT and the surfaces are acceptably small;    -   Attachment between the two surfaces, so as to allow relative        movement between those surfaces—in order to absorb thermo        mechanical displacements arising from either CTE differences or        temperature differences; or any other mechanical movement that        may arise in the application conditions;    -   Location of the active (thermally) CNT at hot spots on the        device;    -   Low cost and high volume production, e.g. making the TIM        independently manufacturable from the device and the heat        spreader;    -   Capable of being readily assembled by a reasonably standard        assembly process;    -   Robust and durable components that are not easily damaged during        shipping, handling and assembly: This particularly applies to        any thermal interface material that contains unprotected        delicate structures such as nano- or micro-structures that are        subject to damage; and    -   Assembly processes that are reasonably manufacturable at an        acceptable cost.

It would be desirable to create a new TIM that addresses some or all ofthe problems listed above.

It is especially desirable to manufacture a material in accordance witha method that includes stabilizing nano-scale fibers—prior to applyingthem to a device requiring a TIM—with a stabilizing material to create astabilized fiber assembly. The stabilizing material may be a fillerdisposed amongst the nano-scale fibers. The stabilizing material may bea capping layer added to the fibers. There may be both stabilizingmaterial and at least one capping layer. There may be two cappinglayers, creating a sandwich-like composite system. Each capping layermay be made of two (or more) sub-layers. Advantageously, the cappingmaterial may be a nano- or micro-particle paste that achieves the goalsabove of attaching the TIM, aligning the nano-scale fibers, andimproving thermal conduction. The particles, in turn may be isotropic oranisotropic, and in the case of anisotropic particles, may beanisotropically aligned. Advantageously, the paste may be locatedadjacent to thermal hotspots in the material to be cooled, while voidsin the paste, to improve processing, may be selectively located adjacentspots not expected to get so hot. Thus, for example, voids in the TIMmay be statistically difficult to avoid, but may be selectively disposedin areas where they are tolerated. In some cases, a void may be desired,for example to help maintain a spatially uniform device temperature inspite of regionally varying heat dissipation.

The material thus manufactured is also advantageous—along with variousembodiments and methods of manufacture. Other materials that might beused, analogously to CNTs, include silver or copper nanowires, carboncolumns, and any fiber of a highly thermally conductive metal alloy.

It is also desirable for devices to be manufactured by adding the TIMafter it is first assembled separately from the device requiring thethermal interface.

It is an object of the invention to provide a method of manufacturing athermal interface material, comprising providing a sheet comprisingnano-scale fibers, the sheet having at least one exposed surface; andstabilizing the fibers with a stabilizing material disposed in at leasta portion of a void space between the fibers in the sheet. The detachedsheet having stabilized fibers may be disposed between two layers of anassembly and is adapted to serve as a thermal interface material. Thedetached sheet having stabilized fibers may be compressed between thetwo layers of the assembly and heated. The fibers may be anisotropicallyaligned. The fibers may have axes which are selectively aligned in thedirection of required thermal transport. The fibers may have axes whichare substantially aligned normal to a plane of the sheet. The method mayfurther comprise depositing a film on a surface of the sheet and/orremoving a surface portion of the stabilizing material to expose an endportion of the fibers. For example, the method may comprise applyingfirst and second nano- or micro-particle containing films on respectiveopposite surfaces of the sheet so that the stabilized fibers aresandwiched between the first and second films and/or applying a firstand second nano- or micro-particle containing sub-film on a respectivesurface of the sheet. A sub-film adjacent to the fibers may comprisepalladium.

The method may further comprise etching the stabilizing material on atleast one surface to create at least one etched portion; and applying atleast one respective metallic nano- or micro-particle film to at leastone etched portion. The nano- or micro-particle film may comprisesinterable metallic particles coated with at least one sacrificialorganic material as a shell; the method further comprising heating thefilm to disrupt the shell, and sintering the particles to form asubstantially contiguous matrix. A pattern of the particle film may beselectively established, in which a first portion of the etched sheet iscovered with the particle film and a second portion of the sheet is notcovered with the particle film, the pattern being adapted to fill atleast one expected gap between the thermal interface material and atleast one solid surface.

The sheet may be selectively stabilized by differentially providingstabilizing material in different spatial portions of the sheet, andwherein a portion of the sheet with reduced stabilizing material ispermeable to gasses.

The process for stabilizing may comprise infiltrating the fibers withpolymerizable matrix material mixed with nano- or micro-particles; andpolymerizing the polymerizable matrix material.

The stabilizing process may also comprise compressing the nano-scalefibers along at least one axis to selectively provide an oriented statein which the fibers are selectively oriented along at least oneelongation axis; and adding at least one capping layer to retain thefibers in the oriented state.

The stabilizing material may be provided with a pattern of regularlydistributed voids.

The stabilizing material may be selectively disposed on the sheet by aprocess comprising inkjet printing of polymerizable material and/ordeposition of an aerosol of polymerizable material droplets. Thestabilizing material may comprise micro or nanoparticles.

The stabilizing process may comprise subjecting the sheet to a vacuum;infiltrating the sheet with a polymerizable material; and releasing thevacuum, to thereby push shrink at least a portion of residual voidspaces in the polymerizable material.

It is a further object to provide a method of manufacturing a device,comprising a) providing a thermal interface material comprising: i) aplurality of aligned nano-scale fibers grown separately from the deviceforming a sheet; and ii) at least one stabilizing material forstabilizing the fibers in a substantially aligned orientation parallelto a desired direction of heat transfer; and b) forming a thermallyconductive interface between the thermal interface material and thedevice.

The thermal interface material may comprise at least one capping layerat a surface of the sheet, and the forming comprises causing the cappinglayer to conform to a shape of the device. The capping layer maycomprise palladium. The fibers may comprise carbon nanotubes. Prior toforming, the ends of the substantially aligned nano-fibers on a surfaceof the sheet may be selectively exposed. The exposing may compriseselectively removing a portion of the stabilizing material on a surfaceof the sheet without degrading the ends of fibers within the removedportion of the stabilizing material. The stabilizing material may beremoved by an ablation process.

The capping layer may also be a solder, e.g., a metallic or metalloidmaterial that melts at a temperature below 450 C, and which, whenmelted, wets another metal surface to form a bond when cooled. Soldersare typically metal alloys, often containing tin, copper, zinc, silver,bismuth, indium, antimony and lead as components. The solder may beprovided as a plating, sinterable powder, or dip, and the solder may beprovided on the surface to be soldered to the TIM. In some cases, asolder composition may be provided on each side of the TIM, and thesolder composition may be the same or different on each, based, forexample, on the respective melting points, and materials to be joined.In the case of an excess of solder, the TIM may be formed to provideflow channels to permit, under compression and heat, the excess solderto be removed from the interface. Preferably, these flow channels aredisposed in areas with reduced need for heat transfer. The compressionduring heating permits the fibers extending from a surface of the TIM topierce the solder, and make direct contact with the surface to bethermally interfaced. The solder, in turn, forms a bond with the TIM,holding it in position and conforming to the mating surface, andprovides at least a modest degree of thermal transfer.

It is another object to provide a thermal interface material sheet,comprising a layer comprising anisotropically oriented thermallyconductive nano-scale fibers, the sheet having at least one exposedsurface; a stabilizing material disposed in at least a portion of a voidspace between the fibers in the layer; and a capping layer in directcontact with an end portion of the fibers proximate to an exposedsurface of the sheet, the fibers extending beyond the stabilizingmaterial and into the capping layer. The fibers may comprise carbonnanotubes, the stabilizing material may comprise an organic polymer, andthe capping layer may comprise palladium.

Another object provides a thermal interface material comprising aplurality of nano-scale fibers having an anisotropic orientation,forming a self-supporting sheet; and a stabilizing material forretaining the fibers in their anisotropic orientation within theself-supporting sheet. The fibers may comprise carbon nanotubes, carboncolumns, silver nanowires, copper nanowires, and/or a high thermalconductivity metal alloy.

The sheet may have first and second surfaces, each nano-scale fiberhaving respective first and second ends, the fibers being oriented suchthat a substantial fraction of the fibers have their respective firstends at the first surface and their second ends at the second surface,so that the fibers directly conduct heat between the first and secondsurfaces. At least one capping layer may be provided having a directthermal interface with at least one of the first ends and the secondends of the fibers.

The stabilizing material may comprise a filler between the fibers. Thestabilizing material may comprise a polymer, formed from a polymerizablematerial, such as a monomer, prepolymer, resin, or the like. Thestabilizing material may comprise a polymerizable material and aconcentration of nanoparticles of 20-60% by volume. The stabilizingmaterial may comprise metallic micro- or nano-particles. The stabilizingmaterial may comprise nano- or micro-particles having a thermalconductivity greater than a bulk material of the stabilizing material.

The material may further comprise at least one capping layer disposed ona surface of the sheet, at least a portion of the fibers terminatingwithin the capping layer. The capping layer may comprise metallicmicroparticles which are at least 1 micron in size

The thermal interface material may be adapted to reduce mechanicalstresses that arise between devices coupled by the thermal interfacematerial as compared to a direct bonding of surfaces of the devices. Thethermal interface material may also be adapted to reduce a stressbetween devices cause by at least one of differences in coefficient ofthermal expansion, or differences in temperature.

The material as provided may comprise areas having low modulus ofelasticity, whereby stresses in an assembly composed of a device, a heatsink, and the thermal interface material disposed between respectivesurfaces of the device and the heat sink, are reduced.

The material may be provided in conjunction with a device or between apair of devices, each having a surface, wherein the material isselectively bonded to one or both surfaces to provide bonded portionsand unbounded portions.

The material may be provided such that the stabilization material has alower modulus of elasticity than the fibers; and at least one of fibersand stabilization material are configured to be selectively disposed atdefined hot spots on a device to be cooled, with at least some gapselsewhere, whereby stresses are reduced.

It is another object to provide a method of forming a thermal interfacematerial comprising a plurality of nano-scale fibers having ananisotropic orientation, forming a self-supporting sheet, and astabilizing material for retaining the fibers in their anisotropicorientation within the self-supporting sheet, comprising the steps ofproviding a mat of nano-scale fibers; applying a stabilizing materialprecursor to the mat; subjecting the mat and stabilizing material totemperature and pressure conditions sufficient to produce thestabilizing material and the sheet; removing a portion of thestabilizing material on at least one surface of the sheet whilepreserving protruding free ends of the nano-scale fibers; and cappingthe free ends of the nano-scale fibers to provide a direct thermalinterface therewith. The sheet is preferably placed between twosurfaces, to thereby conduct heat therebetween.

Further objects, advantages, and embodiments will be apparent in thefollowing.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described with reference to the followingfigures, which constitute non-limiting examples:

FIG. 1A is a schematic of a TIM in accordance with the invention;

FIG. 1B is a side view of the TIM of FIG. 1A;

FIG. 1C is a top view of the TIM of FIG. 1A;

FIG. 1D shows an alternative embodiment to FIG. 1A, with four cappinglayers;

FIG. 1E is an alternative embodiment of FIG. 1B;

FIG. 2 shows assembly of a device in accordance with the invention;

FIG. 3 shows VACNT fibers;

FIG. 4 shows a TIM in accordance with the invention sintered between twodevice surfaces;

FIG. 5A shows CNT's aligned along a neutral axis responsive to shearfrom compression;

FIG. 5B shows cutting CNT's perpendicular to the neutral axis; and

FIG. 6 shows a device incorporating a TIM in accordance with theinvention.

FIG. 7 shows a schematic diagram of a periodic element used in models ofthe TIM.

FIG. 8 shows an SEM image (Inverted side view) of vertically alignedcarbon nanotubes (CNT) at 1,800× magnification, wherein the averageheight of the tubes is around 25 microns.

FIG. 9 shows an SEM image of vertically aligned carbon nanotubes at40,500× magnification, wherein the diameter of the tube that is measuredhere is 100 nm.

FIG. 10 shows a plot of nanotube elements modeled with random normaldistribution length variation and with infinite thermal resistance inthe interface (K_(gap)=0.001 W/mK), showing the distribution thermalconductivity convergence.

FIG. 11 shows a graph of the ratio of effective thermal conductivity tothe bulk conductivity plotted against the percentage of area occupied bythe nanotubes on the silicon surface for the normal distribution casewith finite resistance case.

FIG. 12 shows a graph of the ratio of effective thermal conductivity tothe bulk conductivity plotted against the percentage of area occupied bythe nanotubes on the silicon surface for the normal distributioninfinite resistance case.

FIG. 13 shows a graph of the ratio of effective thermal conductivity tothe bulk conductivity plotted against the percentage of area occupied bythe nanotubes on the silicon surface for the uniform distribution finitecase.

FIG. 14 shows a graph of the ratio of effective thermal conductivity tothe bulk conductivity plotted against the percentage of area occupied bythe nanotubes on the silicon surface for the uniform distributioninfinite case.

FIG. 15 shows a plot of the average value of the ratio of effectivethermal conductivity to the bulk conductivity against the percentage ofarea occupied by the nanotubes on the silicon surface for all the fourstatistical models.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of this application the terms “nanometer realm” or“nano-scale” are understood to mean approximately 1-100 nm, andpreferably 1-10 nm. “Nano-particles” will be in this same size range.Micro range will be understood to mean 0.1 to about 10 microns.

FIG. 1A is a schematic diagram of a TIM system in accordance with theinvention. FIG. 1A illustrates nano-scale fibers 101, such as CNT'sstabilized in a matrix 102 and sandwiched between two capping layers103, such as silver nanoparticle paste. The fibers 101 may be verticallyaligned or randomly oriented. There will be some expense associated withgetting the fibers to be aligned, but aligned fibers are expected todemonstrate superior performance, and are therefore preferred. On theother hand, even in an unaligned state, a portion of the fibers 101 willbe oriented normal to the interface plane. The matrix 102 may includeorganic or organic/inorganic hybrid material stabilizing the arrays ornetworks of nano-scale fibers 101. The fibers 101 function asheat-passages for heat flux.

Other materials that might be used as fibers 101, analogously to CNTs,include silver or copper nanowires, carbon columns, and any other highlythermally conductive fiber, for example formed of metal alloys ororganic or mineral materials.

To achieve high bulk thermal conductivity, the fibers 101 preferablyphysically connect the two opposing solid surfaces 401 (shown in FIG.4). Any low conducting interfaces could be detrimental to overallperformance. This preference for physical connection holds true nomatter whether the fillers in use contain nanoparticles (NPs) ormicroparticles, or whether the fibers are nanowires (NWs), or nanotubes.Examples of thermal properties of nanowires are discussed in severalpapers^(13,14). However, at the interfaces between the TIM and solids(such as a die or a heat sink), direct contacts between the nano-scalefibers and solids direct connection is desirable but generallyimpossible to realize, except perhaps if the fibers are grown betweenthe surfaces, in which case the density of fibers may be too low.Therefore, an approximation of physical contact, which achieves thermalperformance approaching that of true physical contact, is preferred.

A deformable buffer layer 103 with good interfacial adhesion with bothfibers 101 and the solid surface 401 (shown in FIG. 4) is thereforeprovided. Fibers 101 are embedded between capping layers 103. Thecapping layers 103 wet the solid surfaces, potentially eliminating voidsdue to surface roughness. The capping layers 103 also may serve toimprove mechanical strength of the TIM, making it more suitable forautomatic processing, including being punched out and/or being pickedup, carried, or placed, during device assembly. The capping layers 103may also further serve a lateral heat dissipation function,perpendicular to the direction of heat transport provided by the fibers101.

The matrix 102 may be, for example, a polymer with included nano- ormicro-particles.

The assembly shown in FIG. 1A will generally have a thickness in therange of 2 to 1000 microns, preferably in the 100 to 150 micron range.

FIGS. 1B and 1E are schematics of a side view. In the embodiment of FIG.1B, the capping layers 104 are patterned. In the embodiment of FIG. 1E,both the matrix 102 and the capping layers 103 are patterned. Byjudiciously placing voids 104 in the capping layers 103, or a differentmatrix material occupying these zones, both the mechanicalcharacteristics and the processability could be improved. In general,the nanoparticle paste which preferably forms the capping layers 103 isexpected to improve thermal conductivity. Therefore paste should be incontact with anticipated hot spots in the circuitry, while voids 104should be placed where the circuitry needs less cooling.

FIG. 1C shows a top view of the TIM of FIGS. 1 a and 1 b, also showingcapping layer 103 patterning and voids 104.

FIGS. 1A, 1B and 1C illustrate a sandwich-like TIM with nano-scalefibers 101 between two capping layers 103 formed of paste. More layersof paste might be used. For instance, the first thin layer 105 would beapplied to the fibers to promote the adhesion of the second layer 106 tothe opposing surfaces as illustrated in FIG. 1D. This figure shows thefibers 101, the polymer matrix 102, a first capping layer 105 and asecond capping layer 106. Preferably the first, thinner capping layer105 is formed of palladium deposited from spattering or wax processingof nanoparticles or microparticles or other items in a paste that willgo away during processing. There may be a modifier to make the palladiumsoluble in an appropriate solution.

Several different methods of synthesizing nano-scale fibers exist. Inparticular, with reference to CNTs, there are bulk randomly orientedCNTs, random CNTs in a thin mat, and vertically aligned CNTs (VACNT) onsubstrates. Those cover a broad TIM performance range while retaininghigh performance/cost ratios. Because processes such as chemicalpurification and mechanical mixing break CNTs and introduce defects,preferably the skilled artisan will chose as high a quality CNTs as arecurrently available and practicable in terms of cost and meet thefunctional requirements. A number of papers describe methods forsynthesizing aligned CNT^(15,16).

Randomly oriented clean and long CNTs may be synthesized in largequantities using chemical vapor deposition (CVD). The density ofas-synthesized CNT powders can be as low as 30 mg/cm³, which can betuned to optimize the eventual density in composites and the convenientincorporation of matrix materials. Random CNT mats may be obtainedthrough known methods¹⁷.

In addition to the quality of CNTs, density and thickness arecharacteristics of CNT mats. Synthesis conditions generally control thedensity CNT mat and thickness, which is generally achievable in therange of tens of microns.

High density VACNTs of controlled thickness at the vicinity of 10microns may also be synthesized.

Stabilization:

Fibers, such as CNT networks or arrays, can be stabilized byinfiltrating the fibers with a filler, such as monomers or mixture ofmonomers and nanoparticles (NPs) or microparticles followed bypolymerization. Preferably, the fibers are placed in an evacuatedchamber to allow entry of the monomers, which otherwise do not easilywet the fibers. The chamber is then ventilated to push the monomersfurther into the fibers. Voids around fibers are then filled withpolymer. This polymer then leaves fiber configuration intact, whether itis an entangled network or aligned tubes. Alternatively, monomers may bepushed in by filtration. In the latter case, NPs loaded in monomers areretained in fibers and accumulated to high concentrations. Highconcentration means 20-60% by volume. High volume fraction of metallicnano- or micro-particles in the matrix allows for formation ofinterconnected thermal passages upon NP fusing. The monomers are thenpolymerized. Hence, the thermal conductivity of the matrix is enhancedgreatly. The better thermal conductivity is due to the network passageswhich form upon the fusion of high concentration nano- ormicro-particles. Polymerization of monomers provides mechanicalintegrity of the structures. Fracture surface morphology of a VACNTcomposite in accordance with the present process is shown in FIG. 3,showing that the process does not destroy the ordering. Embedded CNTsremain well-aligned. Thus stabilizing the fibers, separate from thedevice requiring a thermal interface, rather than growing the fibers onthe device, allows for more flexibility and lower cost of manufacturing.For example, the fiber mats may be grown under uniform, optimized andtightly controlled conditions, which may be unavailable when seeking togrow the fibers on the device itself.

Orientation of CNTs in polymer composites may also be introduced toinitially randomly-oriented CNT/polymer composite^(9,10,18,19). In thisapproach, composites using bulk CNT powders are compressed biaxiallyfollowed by curing and polymerization. Biaxial confinement deforms CNTnetworks, orienting CNTs along the third, or the neutral axis. Withdeformed CNTs fixed by NP fusion and monomer polymerization, compositefilms obtained by cutting perpendicular to the neutral axis containaligned CNTs, resembling the morphology of a composite film preparedusing VACNTs. FIG. 5A shows CNTs aligned along a neutral axis responsiveto shear from compression. FIG. 5B shows cutting CNT's perpendicular tothe neutral axis. Preferably such compression and cutting will beperformed prior to application of the capping layer or layers. The CNTmay be cut or patterned, for example with a die, laser, water jet,chemical process, optical process, ablative electrical current, or otherknown cutting tool or mechanism. Indeed, the matrix 102 may beselectively processed after polymerization, to weaken it, and thuspermitting a separation.

If CNT mats are used, it is possible to fabricate TIMs with thepatterned matrix and leave regularly arranged voids in composite films.Inkjet printing or aerosol deposition may be used to deliver monomers(with or without NPs or microparticles) to targeted locations. A knownmachine for aerosol deposition is the Optomec M3D printer. Patterningper FIG. 1B is particularly desirable to address the issue of local hotspots, where rapid dissipation in directions both perpendicular andparallel to die surface is required. Patterning is a matter of designoptimization, a balance of performance vs. cost. Patterning could reducethe cost while still get work done, i.e. dissipating heat from a hotspot. For example, use of precious metals used in the capping layers aremay be minimized. Likewise, by reducing the contact area and providingintentional voids, more even contact between the opposed surfaces at theTIM locations may be assured after compressing the TIM between them.

Analogous stabilization can be provided for other nano-scale fibers.

Capping Layer Deposition:

After stabilization with the filler, composite films may be etched usingplasma or reactive ions in order to expose the ends of CNTs or othernano-scale fibers at both surfaces of the film. Therefore, it ispreferred that the exposing step does not substantially degrade thefibers. NP (such as Ag) films of a few microns thick are coated to bothsurfaces of the fiber composite film. Individual NPs are coated withwax-like organic shells, hence NP films are readily deformable underpressure. Seamless joints form between the TIM buffer layers and thesolid surfaces after they are pressed between two solid surfaces at 100°C. A heat treatment at about 100° C. drives away the waxy organicmolecules in the NP shell, triggering the fusion of NPs to form acontiguous metallic layer. This NP layer may therefore be sintered. Thislayer may also conform to the roughness features of the solid surfaces,connecting CNTs or other nano-scale fibers from one solid surface to theother with high thermal conductivity passages, especially when formed insitu between the opposed surfaces to be connected by the TIM. Asubstantial fraction of the fibers are preferably oriented so that theirends are at the surfaces to achieve thermal conductivity. Preferablysubstantially all of the fibers are so oriented, though as discussedabove, the alignment requirement varies in dependence on the applicationand design criteria.

Alternatively, microparticles may be used. While using both filler andcapping layers is expected to enhance mechanical strength and thermalproperties—using one or more capping layers alone, without fillerbetween the fibers, may provide an adequate TIM. Also, the fibersstabilized with filler, and without capping layers, may also provide anadequate TIM.

It is further noted that the process may be asymmetric, with the processaccording to embodiments of the invention provided for only one face ofthe TIM, with other processes used to interface the other face of theTIM with a respective surface.

Using printing (electrostatic, electrophoretic, ink jet, impact) orlithography techniques, capping layers with carefully designed patterns(such as lines, grids, or pads) may be deposited. A design criterion isto deliver just the right amount of materials to fill the gap betweenTIM and solid surfaces and create seamless contact.

FIG. 2 shows a portion of the assembly process of a TIM in accordancewith an aspect of the invention. In this figure, a conveyor 201 carriesfiber composites 202 past an ink jet printer 203, which deposits thenano-particle paste on the fiber composites to form patterns such asshown in FIGS. 1B and 1C.

There are several advantages of fabricating patterned TIM films. Thevoids in the matrix layer serve as breathing channels that release thetrapped air during assembly, and organic molecules during curing NPscapping layers; the voids accommodate thermal expansion cycles andtherefore improve mechanical performance and longevity; and the voidsalso make films more compressible, facilitate the flow of the NP pasteto form better contacts, and reduce the packaging pressure.

Once the TIM is completed, it will preferably be added to a device.Adding the TIM to a device will involve temperature and pressuresufficient to remove excessive (and unintended) voids in the material.

FIG. 4 shows how sintered metal paste 402 helps the TIM bond with devicesurfaces 401. Sandwiched between the capping layers 405 are thenano-scale fibers 403. Each fiber has a first and second end.Substantially all of the fibers 403 are preferably oriented so thattheir first end is at a first device surface and their second end is ata second device surface. The fibers 403 are stabilized with a filler404, while the sintered metal paste 402 serves as the capping layers.The paste has voids 406 which give flexibility and improved stressreduction. The paste 402 is preferably located at anticipated hot spotson at least one of the device surfaces 401.

FIG. 6 shows a device incorporating a TIM in accordance with theinvention. At the top is an air-cooled heat sink 601. Below that is alayer 609 of TIM (TIM2) in accordance with the invention. Below that isa lid 602, which functions as a heat spreader. Below that is anotherlayer 610 of TIM in accordance with the invention (TIM1). Below that isthe chip 608. Below that is a layer of first level interconnect C4 bumps603, also known as flip chip solder bumps, where C4 is the acronym forControlled Collapse Chip Connection. The bumps 603 are interspersed withunderfill 607. The bumps 603 and the underfill 607 rest on a substrate604. The substrate is connected to the Printed Circuit Board (PCB) 606via a second level interconnect 605, similar to elements 603 and 607.

A TIM in accordance with the invention is a hybrid materials system.Preferably this system will include various forms of nanomaterials. ThisTIM will preferably realize one or more of the following goals:

-   -   Low temperature application of the TIM, i.e. below 200° C.,        comparable to the temperature of operation of the device and/or        it electrical assembly, to reduce thermal stress during        operation;    -   Minimizing thermal resistance between two solid surfaces,    -   Usefulness for any applications requiring very high thermal        dissipation by joining two solid surfaces,    -   High bulk thermal conductivity close to theoretical limit of a        composite containing CNTs,    -   Readily deformable surfaces to form intimate contacts with solid        surfaces of varying topological and roughness features,    -   Variable thickness that can be minimized to a few microns,    -   Mechanical robustness for easy processing and application of TIM        as well as long term stability for thermal cycling, and    -   chemical stability therefore environmental and manufacturing        friendliness.

Individual features could be adjusted to optimize the overallperformance of the TIM system.

In addition to heat transport, a TIM should reduce or minimize stressesthat arise between the devices coupled by the TIM. These stresses may bethermo-mechanical in the sense that they are caused by differences incoefficients of thermal expansion between device areas. Stresses mayalso be caused by differences in temperatures in different regions ofthe devices to be coupled. Polymers used in this TIM are preferablychosen to have a low modulus of elasticity. Also, when the TIM isapplied, some areas may be unattached to the devices to be coupled toreduce stresses. Similarly there may be voids in the fibers, filler, orcapping materials to reduce stresses.

During application to a device, a TIM in accordance with the inventionmay be further processed to expose the ends of the nano-particlesimmediately prior to application to a device requiring a thermalinterface. Such further processing might include mechanical means,chemical means, or laser ablation.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the design, manufacture anduse of thermal insulating materials and nano-scale fibers and which maybe used instead of or in addition to features already described herein.Although claims have been formulated in this application to particularcombinations of features, it should be understood that the scope of thedisclosure of the present application also includes any novel feature ornovel combination of features disclosed herein either explicitly orimplicitly or any generalization thereof, whether or not it mitigatesany or all of the same technical problems as does the present invention.The applicants hereby give notice that new claims may be formulated tosuch features during the prosecution of the present application or anyfurther application derived therefrom.

Statistical Model

NOMENCLATURE

-   r,z Cylindrical Coordinates, m-   T Temperature, K-   k_(i) Thermal conductivity of the material i=1, 2, 3, W/mK-   h Convective heat transfer coefficient, W/m²K-   q₀ heat generated per unit volume, W/m³-   q₁ heat flux applied per unit area, W/m²-   q=q₀ L₁+q₁, Effective heat flux, W/m²-   a radius of the lower cylinder, m-   b radius of the middle cylinder, m-   c radius of the upper cylinder, m-   L₁ height of the lower cylinder, m-   L₂ height of the middle cylinder, m-   L₃ height of the upper cylinder, m-   Q heat flow, W-   h_(c) thermal contact conductance, W/K-   ΔT temperature drop across the interface, K-   k_(gap) thermal conductivity of the gap, W/mK-   A_(gap) area occupied by the gap, m²-   L_(gap) length of the gap, m-   R_(gap) resistance due to the gap, K/W-   P Percentage of area occupied by the nanotubes on silicon-   A Area of the thermal interface material layer, m²-   L Thickness of the thermal interface layer, m-   R_(eff) Effective thermal resistance of the thermal interface layer,    K/W-   K_(eff) Effective thermal conductivity of the thermal interface    layer, W/mK-   K_(bulk) Thermal conductivity of the nanotubes, W/mK    Subscripts-   ∞ ambient-   i 1, 2, 3 . . .

An analysis was conducted of the TIM system for configurations in whichthe thickness of the heat source is also taken into account. Bothspecified heat generation and specified uniform heat flux can be appliedto the system. The details of the analytical solution are given in Desaiet al.²¹, expressly incorporated herein by reference. In Desai et al.²²,expressly incorporated herein by references, numerical and analyticalmodels are built for a periodic element (or a unit cell element) of thesystem of vertically aligned nanotubes between silicon and aluminumsurfaces. The size of the periodic element is determined by the size ofthe nanotubes, and the percentage of area they occupy on the siliconsurface (assuming they are uniformly distributed on the siliconsurface). The size of the silicon surface is 1cm×1 cm. The periodicelement is assumed to be cylindrical. FIG. 7 represents one suchperiodic element.

As can be seen from the micrographs shown in FIGS. 8 and 9, thevertically aligned nanotubes grown on a silicon substrate do not havethe same height. To take into account the size variation and to analyzethe effect of this variation on the effective thermal conductivity ofthe system, a statistical approach is applied. An analytical solutionpresented in Desai et al.²¹ is used along with a random number generatorto represent variations in heights of nanotubes over the chip area. Astatistical analysis may then be carried out on the different heights ofthe tubes and a corresponding temperature drop calculated for thatsystem (combination of many unit cells). The results obtained indicatethat considering a small system is sufficient to accurately model theeffect of variation of height over the chip area.

In practice, the nanotubes are grown off a surface (silicon) and theheight to which the nanotubes grow cannot be controlled with greatprecision. Hence, there will be a small gap between some of thenanotubes and the aluminum interface. The analytical solution from Desaiet al.²¹ may be used for modeling a unit cell as shown in FIG. 7. Thevariation in height is accounted for by taking the resultant temperaturedrop in the gap between the end of the nanotube and the aluminum surfacein short tubes and applying the same as an interface temperature drop,as given by relation (1) below.

$\begin{matrix}{R_{gap} = \frac{L_{gap}}{k_{gap}A_{gap}}} & (1)\end{matrix}$

Then,ΔT_(gap)=QR_(gap),  (2)

where k_(gap) is the thermal conductivity of the gap, A_(gap) is thearea occupied by the gap, and L_(gap) is the length of the gap. Q is theheat flowing through the nanotube. This model is then coupled with arandom number generator, which assigns a height to the tubes randomly,and results obtained for a series of interations. The thermal resistanceof each of the nanotubes is stored. The effective resistance of thethermal interface layer is calculated by combining the individualresistances in parallel. The effective resistance is then used toevaluate the one-dimensional effective thermal conductivity of the TIMlayer using the relation,

$\begin{matrix}{k_{eff} = {\frac{L}{R_{eff}A}.}} & (3)\end{matrix}$The result is a model of many vertical nanotubes to form a miniatureversion of the TIM system.

Two different random distributions are considered. First is a normalrandom distribution with mean as the mean height of the nanotubes andstandard deviation σ=1 micron. The second distribution is a uniformrandom distribution, which generates random numbers whose elements areuniformly distributed in the range of the mean, +/−3 micron. The resultsare compared for these two random distributions.

Two different analyses are considered for modeling the effects of heightvariation across the thermal interface material. In the first analysisit is assumed that the nanotubes which are smaller than the mean heightdo not contribute to the effective thermal conductivity (i.e., theresistance of the matrix is very high so there is essentially no heatflowing through these tubes, K_(gap)=0.001 W/mK). The second analysisuses equation (1) to determine the resistance of the short tube(K_(gap)=4 W/mK), and then uses the resistance of the matrix materialand the spreading resistance of the tube with the matrix material added.In the second case the short tubes also contribute to the effectiveconductivity calculation. This results in four different cases:

1) Normal finite—Normal random distribution with finite resistance forthe short tube. In this case, short tubes contribute to the effectivethermal conductivity of the TIM.

2) Normal infinite—Normal random distribution with infinite resistancefor the short tube. So that short tubes do not contribute to theeffective thermal conductivity of the TIM.

3) Uniform finite—Uniform random distribution with finite resistance forthe short tube. In this case, short tubes also contribute to theeffective thermal conductivity of the TIM.

4) Uniform infinite—Uniform random distribution with infinite resistancefor the short tube, so that short tubes do not contribute to theeffective thermal conductivity of the TIM.

FIG. 10, shows a plot of number of runs (same as the number of unitcells used in the model) versus the effective thermal conductivity ofthe matrix for a normal distribution with infinite resistance of theshorter nanotubes. 300 Iterations were required to obtain the requiredconvergence for case 2. In the other cases similar convergence analyseswere performed. For case 1, 300 iterations gave a converged solution.For case 3, 200 iterations and, for case 4, 100 iterations gave aconverged solution.

TABLE 1 Normal distribution effective conductivity values as a functionof percentage of area occupied for finite resistance case. K_(bulk) W/mK2500 1000 500 P K_(eff)/K_(bulk) K_(eff)/K_(bulk) K_(eff)/K_(bulk) 100.063 0.071 0.079 30 0.169 0.185 0.201 50 0.272 0.286 0.317

Table 1 and FIG. 11 are the results obtained for the normal distributionwith finite effective thermal conductivity analyses. The effectivethermal conductivity is scaled with the bulk thermal conductivity of thenanotubes and is plotted against the percentage of area occupied by thenanotubes. The results indicate that taking the average of more than sixruns (three lines shown in the plot) would result in the three linesshown here collapsing into a single line.

The results for normal distribution with infinite resistance arepresented in Table 2 and FIG. 12. Plotting the dimensionless thermalconductivity against the percentage of area occupied by the nanotubesresults in three lines that lie nearly on top of each other, convergingall the data into a single line.

TABLE 2 Normal distribution infinite effective conductivity values as afunction of percentage of area occupied. k_(bulk) W/mK 2500 1000 500 Pk_(eff)/k_(bulk) k_(eff)/k_(bulk) k_(eff)/k_(bulk) 10 0.05 0.048 0.04930 0.148 0.149 0.149 50 0.251 0.248 0.25

Table 3 and FIG. 13 show the results obtained for the uniformdistribution with finite effective thermal conductivity analyses. Theeffective thermal conductivity is scaled with the bulk thermalconductivity of the nanotubes and plotted against the percentage of areaoccupied by the nanotubes. The results indicate that taking average ofmore than six runs (three lines shown in the plot) would result in thethree lines shown here collapsing into a single line.

TABLE 3 Uniform distribution finite effective conductivity values as afunction of percentage of area occupied. K_(bulk) W/mK 2500 1000 500 Pk_(eff)/k_(bulk) k_(eff)/k_(bulk) k_(eff)/k_(bulk) 10 0.06 0.0637 0.071330 0.15 0.172 0.183 50 0.26 0.272 0.288

The results for uniform distribution with infinite resistance arepresented in Table 4 and FIG. 14. In FIG. 15, the collapsed single line(linear fit line through all the lines in case of infinite resistancecase and the centre line in case of the finite resistance case) in eachcase is plotted against the percentage of area occupied by nanotubes.The two lower lines lying on top of each other in FIG. 15 are the linesfor infinite resistance case with normal and uniform distributions. Theylie nearly on top of each other, as in both cases there are 50% of thetubes that are longer or of equal height as the gap and they contributeto the effective thermal conductivity and the other 50% do notcontribute at all. In the uniform distribution with finite resistancethe tubes shorter than the gap height contribute to the effectiveconductivity and hence this gives a higher effective thermalconductivity then the infinite resistance case. In the normaldistribution with finite resistance case the tubes shorter than the gapheight contribute to the effective conductivity with a tighterdistribution of the height of the nanotubes towards the mean (normaldistribution with σ=1) and hence this gives a higher effective thermalconductivity then the uniform distribution with finite resistance case.

TABLE 4 Uniform distribution infinite resistance case effectiveconductivity values as a function of percentage of area occupied.K_(bulk) W/mK 2500 1000 500 P k_(eff)/k_(bulk) k_(eff)/k_(bulk)k_(eff)/k_(bulk) 10 0.048 0.048 0.05 30 0.15 0.15 0.151 50 0.25 0.250.244

The results indicate that the normal distribution with finite resistanceof short nanotubes case give the highest thermal conductivity of all thefour cases. Also, a parametric analysis is carried out by varying thethermal conductivity of the nanotubes and the percentage of area theyoccupy on the silicon surface. By scaling the thermal conductivity withthe bulk conductivity and plotting this against the percentage of areaoccupied, all the lines converge into a single line. The resultsindicate that, despite the effects of height variation, a thermalinterface material with vertically aligned carbon nanotubes has thepotential to be a high thermal conductivity thermal interface material.

The word “comprising”, “comprise”, or “comprises” as used herein shouldnot be viewed as excluding additional elements. The singular article “a”or “an” as used herein should not be viewed as excluding a plurality ofelements. The word “or” should be construed as an inclusive or, in otherwords as “and/or”.

REFERENCES

[1] National Electronics Manufacturing Initiative (NEMI) Roadmap, 2005.

[2] Interface Material Selection and a Thermal Management Technique inSecond-Generation Platforms Built on Intel® Centrino™ Mobile Technology,Intel Technology Journal, E. C. Samson et al., Vol. 9, Issue 1, pp.75-86, February 2005.

[3] S. Berber, Y. K. Kwon, and D. Tomanek, “Unusually high thermalconductivity of carbon nanotubes,” Phys. Rev. Lett., vol. 84, pp.4613-4617, 2000.

[4] S. Maruyama, “A molecular dynamics simulation of heat conduction ofa finite length single-walled carbon nanotube,” Microsc. Thermophys.Eng., vol. 7, pp. 41-50, 2003.

[5] J. W. Che, T. Cagin, and W. A. Goddard, “Thermal conductivity ofcarbon nanotubes,” Nanotechnology, vol. 11, pp. 65-69, 2000.

[6] P. Kim, L. Shi, A. Majumdar, and P. L. McEuen, “Thermal transportmeasurements of individual multiwalled nanotubes,” Phys. Rev. Lett.,vol. 87, no. 21, pp. 215502-1-215502-4, November 2001.

[7] S. U.S. Choi, Z. G. Zhang, W. Yu, F. E. Lockwood, and E. A. Grulke,“Anomalous thermal conductivity enhancement in nanotube suspensions,”Appl. Phys. Lett., vol. 79, pp. 2252-2254, 2001.

[8] M. J. Biercuk, M. C. Llaguno, M. Radosavljevic, J. K. Hyun, A. T.Johnsond, and J. E. Fischer, “Carbon nanotube composites for thermalmanagement,” Appl. Phys. Lett., vol. 80, pp. 2667-2769, 2002.

[9] M. Moniruzzaman, K. I. Winey, “Polymer nanocomposites containingcarbon nanotubes”, Macromolecules, 39 (16): 5194-5205, 2006.

[10] Q. Ngo, B. A. Gurden, A. M. Cassell, G. Sims, M. Meyyappan, J. Li,and C. Y. Yang, “Thermal interface properties of cu-filled verticallyaligned carbon nanofiber arrays,” Nano Lett., vol. 4, pp. 2403-2407,2004.

[11] J. Xu and T. S. Fisher, “Enhancement of thermal interface materialswith carbon nanotube arrays,” Int. J. Heat Mass Transf, vol. 49, pp.1658-1666, 2006, ibid, “Enhancement of thermal contact conductance usingcarbon nanotube arrays,” IEEE Trans. Comp. Packg. Tech. vol. 29, pp.261-267, 2006.

[12] H. Huang, C. Liu, Y. Wu, S. Fan, “Aligned carbon nanotube compositefilms for thermal management”, Adv. Mater, 17, pp. 1652-1656, 2005,ibid, “Effects of surface metal layer on the thermal contact resistanceof carbon nanotube arrays”, APPL. PHYS. LETT. 87, 213108, 2005.

[13] Tian, Weixue; Yang, Ronggui, “Effect of interface scattering onphonon thermal conductivity percolation in random nanowire composites,”90 Applied Physics Letters 26: Art. No. 263105 Jun. 25, 2007.

[14] Yang R G, Chen G, Dresselhaus M S, “Thermal conductivity of simpleand tubular nanowire composites in the longitudinal direction”, 72Physical Review B 12: Art. No. 125418 SEPTEMBER 2005.

[15] Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush, M. P. Siegaland P. N. Provincio, “Synthesis of large arrays of well-aligned carbonnanotubes on glass” 282 Science 1105 (1998).

[16] Z. W. Pan, S. S. Xie, B. H. Chang, C. Y. Wang, L. Lu, W. Liu, W. Y.Zhou, W. Z. Li, and L. X. Qian, “Very long carbon nanotubes” 394 Nature631 (1998).

[17] Coquay, P., De Grave, E., Peigney, A., Vandenberghe, R. E. &Laurent, “C. Carbon nanotubes by a CVD method. Part I: Synthesis andcharacterization of the (Mg, Fe)O catalysts,” 106 Journal Of PhysicalChemistry B 13186-13198 (2002)., ibid, “Carbon nanotubes by a CVDmethod. Part II: Formation of nanotubes from (Mg, Fe)O catalysts,” 106Journal Of Physical Chemistry B 13199-13210 (2002).

[18] Fischer J E, Zhou W, Vavro J, et al. “Magnetically aligned singlewall carbon nanotube films: Preferred orientation and anisotropictransport properties,” JOURNAL OF APPLIED PHYSICS 93 (4): 2157-2163 FEB15 2003.

[19] Garcia E J, Hart A J, Wardle B L, Slocum A H, “Fabrication andnanocompression testing of aligned carbon-nanotube-polymernanocomposites,” 19(16) Advanced Materials 2151-+Aug. 17, 2007.

[20] Iijima S, “Helical microtubules of graphitic carbon”, Nature,Volume 354, Pages 56-58, 1991.

[21] Anand Desai, James Geer, and Bahgat Sammakia, “Models of SteadyState Heat Conduction in Multiple Cylindrical Domains”, Journal ofElectronic Packaging, Volume 128, Number 1, Pages 10-17.

[22] Anand Desai, James Geer, and Bahgat Sammakia, “An Analytical Studyof Transport in a Thermal Interface Material enhanced with Carbonnanotubes”, Journal of Electronic Packaging, Volume 128, Number 1, Pages92-97.

What is claimed is:
 1. A thermal interface material comprising a sheet comprising anisotropically oriented thermally conductive nano-scale fibers, the nano-scale fibers of the sheet being organized to be self-supporting and having at least one exposed surface; a stabilizing material comprising thermally conductive microparticles or nanoparticles in a matrix, disposed in at least a portion of an inter-fiber void space between the nano-scale fibers in the sheet, effective to maintain the anisotropic orientation of the thermally conductive nano-scale fibers; a capping layer in direct contact with an end portion of the fibers proximate to an exposed surface of the sheet, the fibers extending beyond the stabilizing material and into the capping layer wherein the fibers comprise at least one of carbon nanotubes, carbon columns, silver nanowires, copper nanowires, a high thermal conductivity metal alloy, and the stabilizing material comprises an organic polymer; and a device having a surface wherein the exposed surface of the sheet is selectively bonded to the device surface to provide a pattern of bonded and unbonded portions of the exposed surface of the sheet are slidable against the device surface without substantially inducing shear stress.
 2. The thermal interface material according to claim 1, wherein the fibers comprise carbon, and the capping layer comprises palladium.
 3. The thermal interface material according to claim 1, wherein the fibers comprise carbon, and the capping layer comprises a solder.
 4. The thermal interface material according to claim 1, wherein the stabilization material has a lower modulus of elasticity than the fibers; and at least one of sheet and stabilization material are configured to be selectively disposed at predefined hot spots on a device to be cooled, with at least some gaps elsewhere, whereby stresses are reduced.
 5. The thermal interface material according to claim 1, wherein the sheet has first and second surfaces, and each nano-scale fiber has respective first and second ends, the fibers being oriented such that a substantial fraction of the fibers have their respective first ends at the first surface and their second ends at the second surface, so that the fibers directly conduct heat between the first and second surfaces.
 6. The thermal interface material according to claim 5, further comprising at least one capping layer having a direct thermal interface with at least one of the first ends and the second ends of the fibers.
 7. The thermal interface material according to claim 1, wherein the stabilizing material comprises a polymerizable material having a concentration of nanoparticles of 20-60% by volume, a thermal conductivity of the nanoparticles being greater than a thermal conductivity of the polymerizable material after polymerization.
 8. The thermal interface material according to claim 7, wherein nano -scale fibers have respective ends generally aligned toward opposite surfaces of the sheet, the ends on at least one surface selectively extending beyond the stabilizing material into a capping layer, the capping layer comprising metallic microparticles which are at least 1micron in size.
 9. The thermal interface material according to claim 1, wherein the thermal interface material is configured to reduce mechanical stresses that arise between devices thermally and mechanically coupled by the thermal interface material, as compared to a direct bonding of surfaces of the devices.
 10. The thermal interface material according to claim 9, wherein the thermal interface material is configured to reduce a stress between devices caused by at least one of differences in coefficient of thermal expansion, and differences in temperature.
 11. A thermal interface material for cooling a device having a surface comprising a sheet comprising anisotropically oriented thermally conductive nano-scale fibers, the nano-scale fibers of the sheet being organized to be self-supporting and having at least one exposed surface; a stabilizing material comprising thermally conductive microparticles or nanoparticles in a matrix, disposed in at least a portion of an inter-fiber void space between the nano-scale fibers in the sheet, effective to maintain the anisotropic orientation of the thermally conductive nano-scale fibers; wherein the stabilization material has a lower modulus of elasticity than the fibers; and the sheet and stabilization material are selectively disposed at predefined hot spots on the surface of the device to be cooled, with at least some gaps elsewhere at which the sheet and the stabilization material are not disposed at the surface of the device to be cooled, whereby stresses are reduced; a capping layer in direct contact with an end portion of the fibers proximate to an exposed surface of the sheet, the fibers extending beyond the stabilizing material and into the capping layer wherein the fibers comprise at least one of carbon nanotubes, carbon columns, silver nanowires, copper nanowires, a high thermal conductivity metal alloy, and the stabilizing material comprises an organic polymer.
 12. A thermal interface material comprising a plurality of nano-scale fibers having an anisotropic orientation organized to form a self-supporting sheet of nano-scale fibers; and a stabilizing material comprising thermally conductive microparticles or nanoparticles in a matrix formed in an inter-fiber space , which retains the fibers in their anisotropic orientation within the self-supporting sheet; a capping layer in direct contact with an end portion of the fibers proximate to an exposed surface of the sheet, the fibers extending beyond the stabilizing material and into the capping layer wherein the fibers comprise at least one of carbon nanotubes, carbon columns, silver nanowires, copper nanowires, a high thermal conductivity metal alloy, and the stabilizing material comprises an organic polymer; further comprising a device having a surface, wherein the thermal interface material is selectively bonded to the surface of the device to provide bonded portions at which the thermal interface material is bonded the surface of the device and unbonded portions at which the thermal interface material is unbonded and slidable with respect to the surface of the device.
 13. The material of claim 12, wherein the fibers comprise carbon nanotubes.
 14. The material of claim 12, wherein the fibers comprise carbon columns.
 15. The material of claim 12, wherein the fibers comprise silver nanowires.
 16. The material of claim 12, wherein the fibers comprise copper nanowires.
 17. The material of claim 12, wherein the fibers comprise a high thermal conductivity metal alloy.
 18. The material of claim 12, wherein the sheet has first and second surfaces, and each nano-scale fiber has respective first and second ends, the fibers being oriented such that a substantial fraction of the fibers have their respective first ends at the first surface and their second ends at the second surface, so that the fibers directly conduct heat between the first and second surfaces.
 19. The material of claim 18, further comprising at least one capping layer having a direct thermal interface with at least one of the first ends and the second ends of the fibers.
 20. The material of claim 12, wherein the stabilizing material comprises 1-10nm diameter nanoparticles between the fibers.
 21. The material of claim 12, wherein the stabilizing material comprises a polymerizable material and a concentration of nanoparticles of 20-60% by volume.
 22. The material of claim 12, wherein the stabilizing material comprises metallic micro- or nano- particles.
 23. The material of claim 12, wherein the stabilizing material comprises nano- or micro- particles having a thermal conductivity greater than a bulk material of the stabilizing material.
 24. The material of claim 12, further comprising at least one capping layer disposed on a surface of the sheet, at least a portion of the fibers terminating within the capping layer.
 25. The material of claim 24, wherein the capping layer comprises metallic microparticles which are at least 1micron in size.
 26. The material of claim 12, wherein the thermal interface material is configured to reduce mechanical stresses that arise between devices coupled by the thermal interface material as compared to a direct bonding of surfaces of the devices.
 27. The material of claim 26, wherein the thermal interface material is configured to reduce a stress between devices cause by at least one of differences in coefficient of thermal expansion, or differences in temperature.
 28. The material of claim 12, wherein the thermal interface material comprises areas having low modulus of elasticity, whereby stresses in an assembly composed of a device, a heat sink, and the thermal interface material disposed between respective surfaces of the device and the heat sink, are reduced.
 29. The material of claim 12, wherein: the stabilization material has a lower modulus of elasticity than the fibers; and at least one of fibers and stabilization material are configured to be selectively disposed at defined hot spots on a device to be cooled, with at least some gaps elsewhere, whereby stresses are reduced. 